The present invention relates to a microwave plasma source for forming a film on a target by sputtering in, for example, a semiconductor process technique or a surface treatment.
In a sputtering apparatus, an object on which a thin film is to be formed and a target are accommodated in a vacuum chamber maintained in a vacuum condition, and a voltage is applied to the target to cause a discharge in the vacuum chamber to generate gas ions. The electric field allows the gas ions to be accelerated and collide with the target.
The target atoms liberated from the target are deposited on the surface of the object to form a thin film. The gas ions colliding with the target are generated by supplying inert gas such as argon gas and then causing glow discharge in the vacuum chamber.
Recently, a sputtering apparatus having excellent efficiency in forming a thin film has been provided in which high density plasma generated by orthogonal electromagnetic fields collide with a target. This is called a magnetron sputtering apparatus. In this apparatus, permanent magnets are arranged so that a magnetic field generated perpendicularly to the applied electric field to allow the plasma generated from inert gas to be of high density in accordance with the electromagnetic field. Such a magnetron sputtering apparatus allows the deposition speed of a thin film to become high under a relatively lower gas pressure or a relatively lower applied voltage.
Generally, the amount of the inert gas supplied in the vacuum chamber, i.e. the gas pressure, greatly affects the characteristics of a thin film formed by the sputtering.
The higher the gas pressure becomes, the larger are the amount of gas ions generated in the vacuum chamber.
Thus, the amount of the ions colliding with the target increases and then the deposition speed of the material of the target to the surface of the object, i.e. the efficiency of forming a thin film, becomes higher.
However, high gas pressure causes the amount of impurity mingled in the thin film to increase. The reason for this increase in impurity is that when there is a lot of inert gas in the vacuum chamber, the inert gas is mingled as impurity in the thin film depositing on the surface of the object.
Therefore, it is impossible to increase both the efficiency of forming a thin film and the purity of a thin film in the conventional sputtering apparatus.